Good question. 

It would depend on you more than anything else. Since the other characters have attended the same military academy, the assumption would be that your character did not, as he or she is an aggressor. This means that your character would have separated from the rest of the group at age eight or nine. This would leave the aggressor character quite disconnected from the group, but not forgotten.

Another thing would be that this character would not appear at the beginning of the story. He or she would have to appear at a later point, which is impossible to predict when that would be, i.e., how long you would have to wait to actually post something. In fact, it would not make sense for him or her to join the others until somewhere in the middle of the story. 

Another question would be how this character's relationship with everyone else would be. The term 'aggressor' has negative connotations. They do not have to be 'evil,' but they usually do more bad things than good. However, this would not hinder the story all that much.

So, it's a nice idea—and you can do it if you want to—but I strongly advise against it, as you would potentially have to wait a long time before being able to join the story/roleplay. That is my only concern.